Nitric acid conventionally is produced by use of the Ostwald process, in which ammonia is entrained with air and oxidised to form nitrogen monoxide (NO). The nitrogen monoxide is then further oxidised to form nitrogen dioxide (NO2) and its dimer dinitrogen tetroxide (N2O4), which are reacted with and are absorbed by water to produce a solution of nitric acid (HNO3). These reactions can be represented by the following (simplified) chemical equations:4NH3+5O2→4NO+6H2O  (1)2NO+O2→2NO2N2O4  (2)3NO2+H2O→2HNO3+NO  (3)Reaction (2) is the rate limiting reaction in the Ostwald process and this is accommodated by allowing the nitrous gases (i.e., those involved in reactions (2) and (3)) to react for a substantial period of time within an absorption tower whilst adding water to ensure that substantially all of the nitrogen monoxide produced in reaction (1) reacts to produce the nitric acid. The product acid concentration is typically between 50% and 68% HNO3 (w/w), depending upon the operating pressure of, the number of absorption stages in, and the concentration of nitrous gases entering, the absorption tower. However, the nitrous gases typically are relatively dilute, because of the presence of nitrogen introduced with the air, and, hence, a large tower typically is required to facilitate reaction and absorption. Additionally, reaction (2) proceeds more quickly at lower temperatures and this is facilitated by providing refrigerated cooling within the absorption tower, adjacent its exit, in order to prevent or minimise the escape to the atmosphere of nitrogen oxides.
It has now been recognised by the Inventors that, with substantial modification to the manufacturing process, including providing the oxidant for the oxidation of the ammonia in the form (at least substantially) of oxygen, admission of water ballast prior to the ammonia oxidation stage, and retention of the water ballast throughout the process, an absorption stage may be adopted that utilises heat exchange technology and which obviates the requirement for an absorption tower and its attendant disadvantages.